The present invention relates to improvements of apparatus for feedback control and stabilizing of an aircraft. Here particularly, the invention relates to feedback control systems in aircraft with manual command input, e.g. via the control stick.
Automatic flight controllers in aircraft must be constructed to meet a wide variety of flight conditions. For example, present day high-performance aircraft must have controllers which cover a very large speed range. For example, the controller may have to impart STOL properties upon the craft, but will operate under supersonic conditions a little later. Also, sometimes the craft will cruise at very low altitudes; at other times, the craft will loiter at great heights. Naturally, it is close to impossible to use operating parameters in the flight controller that provides optimum operation over the entire range.
Linear operating controllers with fixed parameters are practically unusable in modern day aircraft. Using plural controllers for different speed ranges changes little, and the change over from one to the other is per se a complication. It is known to render the controller rather independent from specific parameters by using suitable networks of, i.e. nonlinear circuit elements. Nevertheless, the situation remains that a craft reacts relatively slowly at low speeds so that the controller must provide for high gain in the error signal-actuator network. However, such high gain will immediately result in problems as to stability when the craft moves at high speeds, such as supersonic speeds. This is one of the reasons which render linear controllers unsuitable for an aircraft that operates over a wide speed range, even if the network tends to render the operator response independent from the parameters, such as gain in the controller amplifier.
Linear controllers have been replaced by controllers in which the parameters of the automatic feedback system are made dependent, e.g. upon speed. For example, dynamic pressure is measured and introduced as parameter into the controller to vary, e.g. amplifier gain. However, introduction of such additional parameter renders the controller even more complicated. It has to be observed that for reasons of safety the principle of redundancy is used extensively. The controller system includes plural, parallely but independently operating controllers. Accordingly, dynamic pressure transducers are used also in the plurality, including plural function generators, signal multipliers, etc., for introducing dynamic pressure into the several controllers as additional variable. Such multiple controllers should, of course, run parallel in the true sense; each should produce the same actuator output for the same operating (external) conditions and commands. The more variables that are used, the more difficult it is to ensure such parallel operations with consistently similar results. On the other hand, introducing a new parameter renders the system inherently less reliable so that the redundancy may even be required to be increased. It can readily be seen, that this approach of introducing another variable snowballs complications and expenditures.
Another approach has been taken in the so-called self-adapting controller. The controller tracks internally system operation and calculates automatically optimum parameters which are then adjusted accordingly and by automatic internal operation. Such a system avoids the complications resulting from introducing additional external variables, but expenditure is considerable, because the principle of redundancy must be maintained. However, reliability of operation has not yet been obtained to the desired and needed degree.
Electronic automatic control systems have become known (see German printed patent application No. 1,563,883) wherein the control parameters are varied by the error signal. Particularly, input and/or feedback impedance are varied by the error signal. This method has the advantage generally that the response and the period of time needed to reduce the error signal, e.g. by a fixed percentage, depends only on the rate of change of measured and/or command input values. It is believed, that this approach is more promising when used in flight controllers for optimizing their operation.